In 1926, Klein [1]} adapted the Kaluza 5D ansatz to quantum considerations, and hypothesized a compact, microscopic fifth dimension, thereby bringing Planck's constant and quantum considerations into a framework that is typically known as “Kaluza-Klein". Klein's suggestion turned out to be a forced marriage of classical and quantum theory that did not satisfactorily describe 4D physics, but it profoundly influenced the direction of quantum field theory, making compact small dimensions an accepted part of physics, in the Kaluza theory [2]}, and in higher-dimensional theories [3]}. Today, many researchers reflexively think of the Kaluza fifth dimension as compact and microscopic.
